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Researchers find microphones are susceptible to light-based signal injection

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JohnG:
It may that that thermal acoustic power is enough.

If 1% of the lasers 5 mW is converted to absorbed acoustic power, we get 50 uW, which would amount to about 77 dB effective SPL at the microphone, which is fairly loud and would be easily picked up by a cheap microphone.

If 0.01% is converted, you are at 57 dB SPL, which is quiet, but still audible.

Feel free to poke holes at these numbers. It's been a long time since I studied anything acoustic.

Cheers,
John

Kleinstein:
The thermal effect if very low efficiency, as the temperature rise is tiny and thus from the Carnot limit efficiency can not be high. So even the 0.01% sound like high. However the sound source can be very close to the microphone, especially when inside. So I don't think there is a simple conversion to SPL.

The electret inside a microphone can (and likely will) also show pyroelectric effect, not as good as the dedicated IR detectors, but here the laser power can be way higher than thermal IR radiation.

JohnG:
Interesting, thanks. I had no idea what the thermal efficiency would be. Definitely my SPL calculations are not directly applicable, because the sensitivity specs for a microphone include the impedance mismatch between the air and the sensing element, which is usually large and thus will attenuate a conventional sound substantially. If heating the element directly, then this mismatch is avoided and the "effective" sensitivity will be higher. Unfortunately, I do not have a good idea what these numbers are.

I had not thought about the pyroelectric effect.

I wanted to put some numbers out to get an idea of what might be possible.

Cheers,
John

ConKbot:

--- Quote from: Yansi on December 30, 2019, 08:38:37 pm ---I very strongly doubt this can work, with a 5mW laser at 100m distance.

If I would just have any kind of laser at home, I'd try right away.

Strongly doubt any effect on an electret and dynamic mikes.   

Not sure about MEMS, but the effect would also be likely pretty small.

--- End quote ---

A "practical" demonstration, along with some nice die shots of MEMS mics.
https://youtu.be/ozIKwGt38LQ

Shown working though a window, with a telescope as a focusing and alignment device. Not exactly laser-pointer+cell phone sound clips easy sort of level, but not exactly hard either.

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